Add locking to PureISCSIDriver around creating Purity Host objects.
Add locking to PureISCSIDriver around creating Purity Host objects.

This will prevent us from getting conflicting credentials for
connection methods which will result in iSCSI attach operations to fail.

Change-Id: Ia206e7b5ff330c8fccb49ed23a31b120a5c04eb3
Closes-Bug: 1444224

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